Seagoing or marine vessels will, in normal sailing conditions, experience rolling motion due to wave interactions with the vessel, as shown in FIG. 1.
Referring to FIG. 1, and in the context of the present disclosure, the term roll, when applied to a marine vessel 10, is defined as the rotation 20 of the vessel 10 about its longitudinal (i.e. bow 40 to stern 50) axis 30.
Such wave interactions usually take the form of waves impinging on the vessel from a position at some angle to the beam of the vessel. This interaction results in an athwardtships wave component acting in a direction normal to the fore-aft axis of the vessel. This athwardtships wave component of the impinging wave causes the vessel to roll in one direction as the wave passes, and then to roll onto the opposite beam once the crest of the wave has passed and the roll moment is reversed.
There may be exceptional cases where significant roll may be caused by a system of waves encountering the vessel from directly ahead or directly astern or close to these two conditions.
The rolling motion resulting from such wave interactions with the vessel may be extreme under various conditions of excitation and vessel loading. A consequence of such rolling motion is that ship-board equipment may be subjected to damage resulting from high loads, and passengers and crew may be distressed and/or incapacitated.
Consequently, it has become an accepted practice that some classes of vessel are equipped with active stabilization systems to reduce roll. An example of such a system is the use of fins projecting from both sides of the vessel which are controlled to present a hydrodynamic angle of attack to the flow of water past the ship. The angle of the fins results in the generation of a restoring moment that acts to counter the direction of roll experienced by the vessel.
For example, roll induced loads may result in container ships experiencing structural problems for cargo locating equipment. Such loads may cause discomfort and/or seasickness for passengers on cruise ships and ferries. Roll induced loads may cause more serious problems for naval vessels in the disruption of the process of targeting and discharging weapons, and also in restricting the operational envelope for the launch and recovery of aerial or marine vehicles.
Conventional active roll stabilization systems for marine vessels rely on physical sensors detecting the vessel's roll motion before they activate the stabilizers. This means that roll is still experienced on board the vessel.
Current systems controlling the operation of fin stabilizers suffer from the following handicap:
In such conventional systems it is currently necessary to receive data from sensors aboard the vessel which sense the rate and/or magnitude of roll. Sensors of this type can give no useful output until the vessel is already in the process of rolling. The stabilization system cannot, therefore, be activated until the vessel has already built up a rolling inertia. Consequently, the stabilizer system will then be attempting to arrest the motion of a very large moving mass before it can begin to impose a counteracting moment to correct the rolling moment. This takes a significant period of time (in terms of the wave period) which means that the vessel is still subject to the effects of roll.